Highlights from Recent Cancer Literature
Journal: Cancer Research (2012)

Su F, Viros A, Milagre C, Trunzer K, Bollag G, Spleiss O, et al. RAS mutations in cutaneous squamous-cell carcinomas in patients treated with BRAF inhibitors. N Engl J Med 2012;366:207–15.The clinical development of BRAF inhibitors that block the active confirmation of the BRAF kinase has led to high rates of objective responses in the 50% of melanoma patients whose cancers harbor the BRAF V600E mutation. However, 15% to 30% of these patients develop nonmelanoma skin cancers during the treatment. Su and colleagues have found a molecular mechanism for this unexpected side effect—a paradoxical activation of mitogen-activated protein kinase (MAPK) signaling. This phenomenon had already been noticed in cell lines that had oncogenic RAS or upregulated receptor tyrosine kinases. Here, the authors found that 60% of nonmelanoma skin cancers that arose during BRAF inhibitor treatment had RAS mutations, with the most common being HRAS Q61L. BRAF inhibitors accelerated soft agar growth of cells harboring this mutation, with evidence of increased extracellular signal-regulated kinase (ERK) phosphorylation and increased expression of ERK-regulated genes. The HRAS Q61L mutation also drives skin tumor development in 2-stage murine skin carcinogenesis. In this model, topical administration of DMBA [7,12-dimethylbenz(a) anthracene] causes the mutation, and tumors appear after chronic inflammation is stimulated by repeated application of the tumor promoter TPA (12-0-tetradeanoyl-phorbol-13-acetate). BRAF inhibitor treatment markedly accelerated tumor development in mice treated with DMBA TPA. A MAP–ERK (MEK) inhibitor given alongside the BRAF inhibitor prevented its tumor-accelerating actions. This elegant combination of experiments in biopsies, cell lines, and mouse models suggests that preexisting subclinical skin tumors with strong upstream MAPK signaling potential are accelerated by BRAF inhibitors. However, this unwanted side effect may be avoided by addition of a MEK inhibitor to the treatment schedule. This article also highlights the importance of genotyping tumors prior to administering BRAF-selective drugs to patients. This way it would be possible to distinguish patients who are likely to respond positively to a therapy with BRAF inhibitors from those who may develop nonmelanoma skin cancers.Garofalo M, Romano G, Di Leva G, Nuovo G, Jeon YJ, Ngankeu A, et al. EGFR and MET receptor tyrosine kinase–altered microRNA expression induces tumorigenesis and gefitinib resistance in lung cancers. Nature Med 2011;18:74–82.The tyrosine kinase inhibitors (TKI), gefitinib and erlotinib, are currently used for treating individuals with non–small cell lung cancers (NSCLC) because they target EGFR, which is activated in these tumors. Acquired resistance to these TKIs is a vexing problem for the biotechnology industry and clinicians alike. The mechanism that leads to drug resistance is not well understood, although previous studies have implicated the MET oncogene in both de novo and acquired resistance to TKIs. Still, how MET overexpression biologically triggers this process was unknown. To address this concern, Garofalo and colleagues hypothesized that microRNAs that act as master regulators of a plethora of cellular processes, including proliferation, apoptosis, invasion, and epithelial-to-mesenchymal transition (EMT), play a key role in mediating TKI resistance. Using independent short hairpin RNA approaches against EGFR and MET, they identified the microRNAs miR-30b, miR-30c, miR-221, and miR-222 as being induced by both EGF and MET receptor signaling, whereas miR-103 and miR-203 were specifically downregulated by MET. Clinical analyses in a cohort of lung tumors showed a positive association between miR-30b, miR-30c, miR-221, and miR-222, and EGFR-MET expression as well as an inverse correlation between miR-103 and miR-203, and MET. The authors further identified BIM and APAF-1 as common targets of the EGFR-MET controlled microRNA complex that lead to gefitinib resistance. Silencing miR-30 or miR-221 or overexpressing miR-103 or mir-203 induced apoptosis and decreased tumor size in response to gefitinib. Complementary to this finding, MET induced EMT via inhibition of miR-103 and miR-203, and caused enhanced migration by relieving the repressive effect of these microRNAs on their targets, PKC-ϵ and SRC. In totality, these findings suggest that modulation of specific microRNAs by EGFR-MET contributes to gefitinib resistance and therefore represents a novel therapeutic approach for treatment of NSCLCs. This multidimensional approach could have implications for lung cancer therapy, for example by stratifying patients based on MET-microRNA regulon to select those who might benefit from treatment with gefitinib.Kinross KM, Montgomery KG, Kleinschmidt M, Waring P, Ivetac I, Tikoo A, et al. An activating Pik3ca mutation coupled with Pten loss is sufficient to initiate ovarian tumorigenesis in mice. J Clin Invest 2012;122:553–7.The phosphoinositide 3-kinase (PI3K) pathway is one of the most commonly deregulated pathways in cancer. Somatic mutations in a gene encoding the p110 alpha subunit (PIK3CA) enhance PI3K activity and are often found in human cancer. However, the role of increased PIK3CA activity in tumor development and progression remains unknown. Kinross and colleagues have generated the first mouse model harboring an inducible knock-in of the human cancer-associated PIK3CA H1047R mutation (PIK3CAH1047R) to study the contribution of this mutation towards ovarian tumorigenesis. The authors demonstrate that PIK3CAH1047R expression alone is insufficient to induce AKT phosphorylation in vivo, and is only weakly tumorigenic. However, the PIK3CA mutant combined with biallelic PTEN deletion (PTENdel/del) leads to AKT hyperphosphorylation at serine-473, threonine-308, and phosphorylation of RPS6, as well as the formation of high-grade serous adenocarcinomas and granulosa cell tumors. These results are especially significant in light of the fact that PIK3CA mutations are commonly coexpressed alongside PTEN loss or oncogenic KRAS in human ovarian cancer. Importantly, Kinross and colleagues further show that pharmacologically inhibiting the PI3K and mTOR signaling pathways increases the survival rates of PIK3CAH1047R/PTENdel/del mutant mice. The authors' findings have strong clinical relevance for the development of new treatment regimens for human epithelial cancers harboring PIK3CA and PTEN mutations.Bronisz A, Godlewski J, Wallace JA, Merchant AS, Nowicki MO, Mathsyaraja H, et al. Reprogramming of the tumor microenvironment by stromal PTEN-regulated miR-320. Nature Cell Biol 2011 Dec 18. [Epub ahead of print].The functional role of stromal cells such as fibroblasts in tumor development and metastasis is important but poorly understood. Although reports in the literature demonstrate that tumorassociated fibroblasts can provide a favorable environment for cancer initiation and progression, the mechanisms underlying this effect remain unclear. In this article, Bronisz and colleagues uncover a new tumor-suppressive axis in stromal fibroblasts linking PTEN, the microRNA miR-320, and transcription factor v-ets erythroblastosis virus E26 oncogene homolog 2 (ETS2), which is important for reprogramming tumor cells in cancer initiation and progression. The authors used genetic strategies in stromal fibroblasts surrounding the mammary epithelial ducts to compare the expression of microRNAs in mouse mammary fibroblasts (MMF) with and without PTEN. They identified a group of microRNAs that are differentially expressed in PTEN−/− cells compared with wild-type cells. The work of Bronisz and colleagues functionally validates that miR-320 plays a cell–nonautonomous tumor-suppressive role in stromal cells. Specifically, coinjecting breast cancer tumor cell line DB7 with PTEN−/− MMFs resulted in a 4-fold increase in tumor growth compared with DB7 coinjected with PTEN+/+ MMFs. Interestingly, restoration of miR-320 in PTEN−/− cells significantly decreases tumor growth. The authors also show that downregulation of miR-320 in PTEN−/− MMFs influences gene expression of the surrounding epithelial or endothelial tumor cells. They further demonstrate that miR-320 in stromal fibroblasts influences the invasiveness of epithelial tumor cells through the action of secreted factors. Using a proteomics analysis of conditional media of PTEN−/− MMFs expressing either miR-320 or control, Bronisz and colleagues discovered a signature of miR-320 tumor-regulating secretory factors that, when PTEN is lost, act to increase the migration and invasive behavior of epithelial and endothelial tumor cells. The authors further demonstrate that these factors can be categorized into different groups: One group is comprised of proteins that are direct targets of miR-320, and a larger group transcriptionally regulates ETS2, which itself is a target of miR-320. Bronisz and colleagues show that the miR-320 secretome signature can be used to discriminate between normal and tumor stroma in human breast cancer. These findings highlight the functional role of stromal cells in directly initiating cancer or acting as a metastatic switch.Putluri N, Shojaie A, Vasu VT, Vareed SK, Nalluri S, Putluri V, et al. Metabolomic profiling reveals potential markers and bioprocesses in bladder cancer progression. Cancer Res 2011;71:7376–86.It is estimated that 69,250 new cases of bladder cancer and 14,990 deaths from bladder cancer occurred in 2011. The current approach for diagnosing and monitoring bladder cancer is cystoscopy, aided by cytology and biopsy. Urine cytology provides acceptable sensitivity and specificity in the context of high-grade bladder cancer, whereas sensitivity is poor for lowgrade tumors. Development of assays with enhanced sensitivity in permitting disease stratification is important in the management of this disease. Using high-throughput mass spectrometry to measure over 2,000 compounds in 58 clinical specimens, Putluri and colleagues have identified 35 metabolites displaying measurable changes in bladder cancer. These metabolomics signatures in urine provide potential markers for distinguishing bladder cancer from controls and also nonmuscle from muscle-invasive bladder cancer. A potential role for DNA methylation in altering phase I/II xenobiotic metabolism in bladder cancer was suggested by enrichment-based bioprocess mapping. With the use of bisulfite sequence analysis and methylation-specific PCR, in addition to in vitro experiments with T-24 bladder cancer cells treated with 5-aza-2′-deoxycytidine, hypomethylation in the cytochrome P450 1A1 (CYP1A1) and cytochrome P450 1 B1 (CYP1B1) promoters in bladder cancer was identified. Although further studies are required with a much larger cohort of patients, metabolomics signatures described in this article may provide a basis for potentially developing urinary biomarkers for early detection and staging of bladder cancer. Additionally, this study highlights the importance of epigenetic changes in metabolic profiles as potentially underlying factors in determining bladder tumor development and progression.Note: Breaking Advances are written by Cancer Research Editors. Readers are encouraged to consult the articles referred to in each item for full details on the findings described.